It has long been desirable to produce coated cookware which has an inner cooking surface having good release properties. It is also desirable that heat can be rapidly transferred to such cooking surfaces without the need to subject the outer bottom surface of the cooking vessel to excessive heat. Further, it is desirable to uniformly distribute the heat about the entire cooking surface such that food placed on the cooking surface may be evenly cooked. Even heat distribution is particularly problematic in situations in which the cookware is placed on a heating element that has a smaller diameter than the diameter of the cookware's bottom. In such a case, the central region of the cooking surface heats more rapidly and tends to remain hotter than the outer peripheral regions and results in uneven heating of the cookware's contents.
It also has long been desirable to produce coated cookware which has decorative appeal. One attempt to produce patterned cookware which exhibits an illusion of optical depth is described in GB 1,131,038 (Tefal). The specification discloses a process for producing a pattern of flaked magnetic particles in a polytetrafluoroethylene (PTFE) matrix as a coating on a substrate. The process is carried out by mixing the flakes with an aqueous dispersion of PTFE and coating the dispersion onto the substrate. After the coating step, a magnet is placed on the underside of the substrate (base), and the magnetic field from the magnet causes the flakes to be attracted toward the magnet. As shown in FIG. 3 of the '038 patent, this movement includes the vertical and near vertical orientation of the flakes within the coating thickness and the flakes are entirely contained within the coating, which means that their largest dimension is smaller than the thickness of the coating. This requires either thick coatings or very small flakes (small largest dimension). The problem with small flakes, however, is that they tend not to form a distinguishable pattern in the coating. Consequently, thick PTFE coatings are necessary to produce a visible pattern. Even then, the vertical orientation of the flakes by the magnetic lines of force inevitably causes flakes near the top surface of the coating to protrude from the surface, causing roughness of the baked coating, which is undesirable for a release coating. The '038 patent also discloses that the base has cavities in it, i.e., it has a rough surface, which enables the flakes to be immobilized during the baking of the coating. Among the problems with the magnetic patterning of the release coating by the process of the '038 patent is the need for an excessively thick PTFE coating, which nevertheless fails to completely contain all of the flakes within its thickness and the need for a roughened substrate for adhering the coating to the substrate and immobilizing the flakes during sintering.
Another problem with the pattern formed by the process of the '038 patent is that the pattern is "fuzzy", i.e., lacks clarity. When the coated substrate is placed directly on the magnet of FIG. 1 of the '038 patent, the annular pole piece of the magnet is reproduced in the coating as a toroid ring, deviating from the shape of the circular ring of the pole piece serving as the pattern. When a shaped plate is laid across the top of the magnet, the resultant imprint of the shaped plate is especially fuzzy where the magnetic force is directed through the bulk area of the shaped plate as shown in FIG. 2 of the '038 patent. The "fuzzy" image is a manifestation of the of the '038 patent method producing unwanted field lines (magnetic background effects); such method also produces a rough decorative surface. If a stronger magnet is used in the method of the '038 patent, to try to eliminate the fuzziness of the image, i.e. sharpen the image, another unwanted background effect occurs, namely reproduction of the shape of the magnet in the pattern in the coating.
In addition to design, cookware often includes liquid level markings on the inside sidewalls of pots and pans or the like. Traditionally, such markings have been achieved by embossing the metal base prior to overcoating with nonstick finish. However, the depressions protrusions formed by embossing can interfere with the release properties of the surface, causing a buildup of food deposits and becoming a source of corrosion.